German patent 2,827,847 of Hans Joachim Schmidt describes a coupling of the type usable to connect a hose end to a hand shower or the like. The coupling has a male portion with a protruding tubular sleeve and a female portion with a seat shaped to receive the sleeve and provided with a latch mechanism for holding the sleeve in place. Diametrally opposite buttons on the female portion can be depressed to release the latch and allow the two parts to be separated. Such an arrangement makes it fairly simple to switch shower heads, for instance to install one with a different spray pattern.
U.S. Pat. No. 4,030,360 describes a fitting which can be installed immediately upstream of a shower or the like to provide a direct readout of water temperature. This gauge has a female threaded socket on one side and a male threaded nipple on the other so that it can be placed, for instance, immediately upstream of a shower head. Mounting such a bulky device on a hand shower, however, is out of the question, especially combined with the extra bulk of a snap coupling.